Mixing device for a gas burner

ABSTRACT

A mixing device ( 10 ) for a gas burner, with a housing ( 11 ) having a combustion air inlet ( 13 ), a gas inlet ( 14 ) and an outlet ( 15 ) for the mixture of gas and combustion air, and with a Venturi device ( 12 ) which is positioned in the housing, wherein the Venturi device is contoured to form a contraction section ( 19 ), a mixing section ( 20 ) and a diffuser section ( 21 ). A guide device ( 22 ) for combustion air, which divides combustion air which enters the mixing device via the combustion air inlet ( 13 ) into a primary flow and a secondary flow, may be positioned inside the Venturi device, wherein the combustion air of the primary flow, issuing from the combustion air inlet, can be fed to the mixing section ( 20 ) via the contraction section ( 19 ), and wherein the combustion air of the secondary flow, issuing from the combustion air inlet, can be mixed downstream of the mixing section ( 20 ) in the region of the diffuser section ( 21 ) with the mixture of the gas and the combustion air of the primary flow.

The present application claims priority to German Patent Application No.DE 10 2010 010 791.3, filed on Mar. 9, 2010, entitled “MIXING DEVICE FORA GAS BURNER”, which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a mixing device for a gas burner.

BACKGROUND

A mixing device for a gas burner with a housing and a Venturi devicewhich is positioned in the housing is known from DE 296 17 621 U1. Thehousing of the mixing device which is disclosed there has a combustionair inlet, a gas inlet and an outlet for the mixture of gas andcombustion air. The Venturi device which is positioned in the housing isconstructed as a Venturi nozzle and serves for the mixing of thecombustion air and the gas.

A Venturi device, designed as a Venturi nozzle, for a mixing device of agas burner, which is positioned in a housing of a mixing device andformed from an inlet funnel and a discharge funnel, is known from DE 19743 464 C1, wherein the Venturi device, specifically the inlet funnel andthe discharge funnel of the Venturi device, is contoured to form acontraction section, a mixing section and a diffuser section, and insuch a way that the combustion air, issuing from the combustion airinlet, can be fed to the mixing section via the contraction sectionwhich narrows in the direction of the mixing section, and that themixture of gas and combustion air, issuing from the mixing section, canbe fed to the outlet via the widening diffuser section. In the mixingsection, an inlet-side section of the discharge funnel overlaps adischarge-side section of the inlet funnel, forming an annular gap or anannular gap-like nozzle, via which the gas can be admixed with thecombustion air.

For further clarification of the mixing devices of a gas burner, whichare known from the prior art, reference may be made to FIG. 1 whichshows a schematized cross section of a mixing device 10′ which is knownfrom the prior art. The mixing device 10′ according to FIG. 1 comprisesa housing 11′, inside which a Venturi device 12′ is positioned. Thehousing 11′ of the mixing device 10′ has a combustion air inlet 13′ forcombustion air V, a gas inlet 14′ for gas G and also an outlet 15′ forthe mixture M of gas and combustion air. The Venturi device 12′, whichis positioned in the housing 11′, also has the combustion air inlet 13′and also the outlet 15′ for the mixture M of gas and combustion air,wherein the gas G, which enters the housing 11′ via the gas inlet 14′,flows radially outward around the Venturi device 12′ and, via an annulargap 16′ which is formed between an inlet funnel 17′ and a dischargefunnel 18′ of the Venturi device 12′, can be admixed with or added tothe combustion air V. The Venturi device 12′, which includes the inletfunnel 17′ and the discharge funnel 18′, forming a contraction section19′, a mixing section 20′ and a diffuser section 21′, is contoured insuch a way that the combustion air V, issuing from the combustion airinlet 13′ of the housing 10′ or of the Venturi device 12′, can be fed tothe mixing section 20′ via the contraction section 19′ which narrows inthe direction of the mixing section 20′. The mixture M of gas andcombustion air, issuing from the mixing section 20′, can be fed to theoutlet 15′ via the diffuser section 21′, which widens in the directionof the outlet 15′. The annular gap 16′ is located in the mixing section20′. The annular gap 16′ has a cross section which results from adiameter difference between the outside diameter of a discharge-side endof the inlet funnel 17′ and the inside diameter of an inlet-side end ofthe discharge funnel 18′ in the mixing section 20′. The outside diameterof the annular gap 16′ thus corresponds to the inside diameter of theinlet-side end of the discharge funnel 18′ and therefore to the diameterof the mixing section 20′.

A fan or a blower is typically associated with the outlet 15′ or withthe combustion air inlet 13′ of such a mixing device of a gas burner,wherein a speed of the fan or of the blower determines the quantity orthe volumetric flow of the mixture of gas and combustion air which isfed to the gas burner. The ratio of gas and combustion air in thecombustion air/gas mixture is meant to be relatively constant and isprimarily determined by a ratio of the cross section of the inlet funnel17′ in the mixing section 20′ of the Venturi device 12′ and the crosssection of the annular gap 17′ which is formed between the inlet funnel17′ and the discharge funnel 18′ of the Venturi device 12′. Regardlessof the speed of the fan, the ratio of gas and combustion air in thecombustion air/gas mixture is supposed to be constant, wherein withincreasingly or reducing speed of the fan, undesirable deviations in theratio of gas and combustion air develop on account of inaccuracies withrespect to control engineering. In the case of known mixing devices, thespeed of a fan which interacts with the mixing device can be reducedonly to a certain extent while maintaining the desired ratio of gas andcombustion air. This ultimately results in a realizable modulation rangeof between 1 and 5, in which a high modulation quality, specifically adesired ratio of gas and combustion air in the combustion air/gasmixture, can be provided. A modulation of 1 corresponds to a full loadspeed of the fan and a modulation of 5 corresponds to 20% of the fullload speed of the fan. With mixing devices which are known from theprior art, the speed of the fan which interacts with the mixing devicecan therefore be reduced only to 20% of the full load speed of the fanwhile maintaining the desired ratio of gas and combustion air.

SUMMARY

This disclosure relates to a new type of mixing device for a gas burner,which on the one hand has a low flow resistance and on the other handenables a broader modulation range while helping to ensure a highmodulation quality.

In one illustrative embodiment, a guide device for combustion air, whichdivides combustion air which enters the mixing device via the combustionair inlet into a primary flow and a secondary flow, is positioned insidethe Venturi device, wherein the combustion air of the primary flow,issuing from the combustion air inlet, can be fed to the mixing sectionvia the contraction section, and wherein the combustion air of thesecondary flow, issuing from the combustion air inlet, can be mixeddownstream of the mixing section in the region of the diffuser sectionwith the mixture of the gas and the combustion air of the primary flow.

In some instances, it may be ultimately possible to have a broadermodulation range between 1 and 10 with good modulation quality for a fanwhich interacts with the mixing device. Using the mixing deviceaccording to this disclosure, the speed of a fan which interacts withthe mixing device may be reduced to 10% (or less) of the full load speedof the fan while maintaining the desired ratio of gas and combustionair. In some instances, the speed of a fan which interacts with thedevice may be reduced to a greater extent than in the case of mixingdevices which are known from the prior art, and this while maintaining adesired ratio of gas and combustion air. Furthermore, a low flowresistance may be accomplished.

The above summary is not intended to describe each disclosed embodimentor every implementation. The Figures, Description and Examples whichfollow more particularly exemplify these embodiments.

BRIEF DESCRIPTION

The disclosure may be more completely understood in consideration of thefollowing description of various embodiments in connection with theaccompanying drawings, in which:

FIG. 1 shows a schematized cross section through a mixing device whichis known from the prior art for mixing gas and combustion air for a gasburner;

FIG. 2 shows a schematized cross section through a mixing device formixing gas and combustion air for a gas burner according to an exemplaryembodiment; and

FIG. 3 shows a schematized cross section through a mixing device formixing gas and combustion air for a gas burner according to anotherexemplary embodiment.

DESCRIPTION

FIG. 2 shows a schematized cross section of a first exemplary embodimentof a mixing device 10, wherein the mixing device 10 includes a housing11, inside which a Venturi device 12 is positioned. The housing 11 ofthe mixing device 10 has a combustion air inlet 13 for combustion air V,a gas inlet 14 for gas G, and also an outlet 15 for the mixture M of gasand combustion air.

The Venturi device 12, which is positioned in the housing 11, also hasthe combustion air inlet 13 and also the outlet 15 for the mixture ofgas and combustion air, wherein the gas G, which enters the housing 11via the gas inlet 14, flows radially outwards around the Venturi device12, which is visible in FIG. 2, and, via an annular gap 16 which isformed between an inlet funnel 17 and a discharge funnel 18 of theVenturi device 12, can be admixed with or added to the combustion air V.

The Venturi device 12, which includes the inlet funnel 17 and thedischarge funnel 18, is contoured to form a contraction section 19, amixing section 20 and a diffuser section 21.

In some instances, a guide device 22 for the combustion air ispositioned inside the Venturi device 12. The guide device 22 divides thecombustion air V, which enters the mixing device 10 or the Venturidevice 12 via the combustion air inlet 13, into a primary flow V₁ and asecondary flow V₂. The combustion air of the primary flow V₁, whichflows around the guide device 22, issuing from the combustion air inlet13, can be fed to the mixing section 20 via the contraction section 19,wherein the combustion air of the primary flow V₁ is mixed in the regionof the mixing section 20 with the gas G which enters the mixing device10 via the gas inlet 14. The combustion air of the secondary flow V₂,which flows through the guide device 22, issuing from the combustion airinlet 13, can be mixed downstream of the mixing section 20 in the regionof the diffuser section 21 with the mixture of the combustion air of theprimary flow V₁ and the gas.

By establishing the secondary flow V₂ of the combustion air, which flowsthrough the guide device 22 and is mixed with the gas and the combustionair of the primary flow V₁ downstream of the mixing section 20, abroader modulation range can be realized, and this while maintaining ahigh modulation quality.

In the exemplary embodiment of the mixing device 10 which is shown inFIG. 2, the guide device 22 is positioned centrally in the Venturidevice 12, and in some cases concentrically. Also, the guide device 22has a tubular contour, specifically a continuously cylindrical contour,in the exemplary embodiment which is shown in FIG. 2, and has a defineddiameter d₂₂ and a defined length l₂₂. In the depicted exemplaryembodiment of FIG. 2, the diameter d₂₂ of the tubular guide device 22 isconstant over the entire length l₂₂ of the device.

The inlet 13 for the combustion air V has a diameter d₁₃ and the outlet15 for the mixture M of gas and combustion air has a diameter d₁₅. Themixing section 20 has a diameter d₂₀.

The ratio d₁₃/d₁₅ between the diameter d₁₃ of the inlet 13 for thecombustion air V and the diameter d₁₅ of the outlet 15 for the mixture Mof gas and combustion air may be, for example, between 0.75 and 1.25,especially (100±25) %. This ratio d₁₃/d₁₅ is preferably 1.0.

The ratio d₂₀/d₁₅ between the diameter d₂₀ of the mixing section 20 andthe diameter d₁₅ of the outlet 15 for the mixture M of gas andcombustion air may be, for example, between 0.25 and 0.75, especially(50±25) %. This ratio d₂₀/d₁₅ is preferably 0.5.

The ratio l₁₉/d₂₀ between the length l₁₉ of the contraction section 19and the diameter d₂₀ of the mixing section 20 may be, for example,between 0.5 and 1.5, especially 1±0.5. This ratio l₁₉/d₂₀ is preferably1.

The ratio l₂₁/d₂₀ between the length l₂₁ of the diffuser section 21 andthe diameter d₂₀ of the mixing section 20 may be, for example, between 2and 6, especially 4±2. This ratio l₂₁/d₂₀ is preferably 4.

As already mentioned, the guide device 22, which may be positioned inthe Venturi device 12, has the diameter d₂₂. The ratio d₂₂/d₂₀ betweenthe diameter d₂₂ of the guide device 22 and the diameter d₂₀ of themixing section 20 may be, for example, between 0.05 and 0.55, especially(30±25) %. This ratio d₂₂/d₂₀ is preferably 0.3.

The ratio l₂₂/d₂₂ between the length l₂₂ of the guide device 22 and thediameter d₂₂ of the guide device may be, for example, between 5 and 15,especially 10±5. This ratio l₂₂/d₂₂ is preferably 10.

The distance x between the flow discharge-side end 24 of the guidedevice 22 and the outlet 15 for the mixture M of gas and combustion airis dimensioned in such a way that the ratio x/d₂₀ between this distancex and the diameter d₂₀ of the mixing section 20 may be between 0 and 2,especially 1±1. The ratio x/d₂₀ is preferably 1.

In the depicted exemplary embodiment of FIG. 2, the flow discharge-sideend 24 of the guide device 22 may lie upstream of the outlet 15 for themixture of gas and combustion air.

The flow discharge-side end 24 of the guide device 22, which in thedepicted exemplary embodiment of FIG. 2 preferably has a cylindricalcontour, can be flared like a funnel.

The flow inlet-side end 23 of the guide device 22 in the depictedexemplary embodiment of FIG. 2 may lie downstream of the combustion airinlet 13. The offset between the flow inlet-side end 23 of the guidedevice 22 and the combustion air inlet 13 preferably corresponds at mostto the diameter d₂₂ of the guide device 22.

In contrast to this, it is also possible, however, that the flowinlet-side end 23 of the guide device 22 terminates flush with thecombustion air inlet 13, or may even extend out past the air inlet 13.

In the exemplary embodiment of FIG. 2, the tubular guide device 22 isshown to be cylindrically contoured, and therefore has a diameter d₂₂which is constant over the entire length l₂₂ of the guide device 22.

In the exemplary embodiment of FIG. 2, the guide device 22 has acircular contour in cross section. In contrast to this, the guide device22 may have an oval or elliptical contour in cross section, wherein thediameter d₂₂ is then the so-called large axis of the respective ellipse.These are only examples, and it is contemplated that the guide device 22may have any suitably shaped contour, as desired.

FIG. 3 shows another exemplary embodiment of a mixing device 110,wherein the mixing device 110 includes a housing 111, inside which aVenturi device 112 is positioned. The housing 111 of the mixing device110 has a combustion air inlet 113 for combustion air V, a gas inlet 114for gas G and also an outlet 115 for the mixture M of gas and combustionair. The Venturi device 112, which is positioned in the housing 111,also has the combustion air inlet 113 and also the outlet 115 for themixture of gas and combustion air, wherein the gas G, which enters thehousing 111 via the gas inlet 114, flows radially outward around theVenturi device 112 which is visible in FIG. 3, and, via an annular gap116 which is formed between an inlet funnel 117 and a discharge funnel118 of the Venturi device 112, can be admixed with or added to thecombustion air V. In the illustrative embodiment, the Venturi device112, which may include the inlet funnel 117 and the discharge funnel118, is contoured to form a contraction section 119, a mixing section120 and a diffuser section 121.

As show in FIG. 3, a guide device 122 for the combustion air may bepositioned inside the Venturi device 112. The guide device 122 maydivide the combustion air V, which enters the mixing device 110 or theVenturi device 112 via the combustion air inlet 113, into a primary flowV₁ and a secondary flow V₂. The combustion air of the primary flow V₁,which flows around the guide device 122, issuing from the combustion airinlet 113, can be fed to the mixing section 120 via the contractionsection 119, wherein the combustion air of the primary flow V₁ is mixedin the region of the mixing section 120 with the gas G which enters themixing device 110 via the gas inlet 114.

The combustion air of the secondary flow V₂, which flows through theguide device 122, issuing from the combustion air inlet 113, can bemixed downstream of the mixing section 120 in the region of the diffusersection 121 with the mixture of the combustion air of the primary flowV₁ and the gas. By establishing the secondary flow V₂ of combustion airwhich flows through the guide device 122 and is mixed downstream of themixing section 120 with the gas and the combustion air of the primaryflow V₁, a broad modulation range can be realized, and this whilemaintaining a high modulation quality.

In the exemplary embodiment of the mixing device 110 which is shown inFIG. 3, the guide device 122 may have a tubular contour and has twosections, specifically a truncated cone-like section 104, which widensout in the flow direction, and a cylindrical section 105, which adjoinsthe truncated cone-like section 104 downstream. The truncated cone-likesection 104 has a length l₁₀₄, a flow inlet-side or upstream diameterd₁₀₂ and a flow discharge-side or downstream diameter d₁₀₁. Thecylindrical section 105 has a length l₁₀₅ and a diameter whichcorresponds to the flow discharge-side or downstream diameter d₁₀₁ ofthe truncated cone-like section 104. The inlet 113 for the combustionair V has a diameter d₁₀₃ and the mixing section 120 has a diameterd₁₀₀.

A ratio l₁₀₄/d₁₀₂ between the length l₁₀₄ of the truncated cone-likesection 104 of the guide device 122 and the flow inlet-side or upstreamdiameter d₁₀₂ of the truncated cone-like section 104 of the guide device122 may be, for example, between 2 and 6, especially 4±2. The ratiol₁₀₄/d₁₀₂ is preferably 4.

A ratio l₁₀₅/d₁₀₂ between the length l₁₀₅ of the cylindrical section 105of the guide device 122 and the flow inlet-side or upstream diameterd₁₀₂ of the truncated cone-like section 104 of the guide device 122 maybe, for example, between 1 and 3, especially 2±1. The ratio l₁₀₅/d₁₀₂ ispreferably 2.

A ratio d₁₀₂/d₁₀₁ between the flow inlet-side or upstream diameter d₁₀₂of the truncated cone-like section 104 of the guide device 122 and theflow discharge-side or downstream diameter d₁₀₁ of the truncatedcone-like section 104 of the guide device 122 may be, for example,between 0.25 and 0.75, especially (50±25) %. This ratio d₁₀₂/d₁₀₁ ispreferably 0.5.

A ratio d₁₀₁/d₁₀₀ between the flow discharge-side or downstream diameterd₁₀₁ of the section 104 of the guide device 122, which corresponds tothe diameter of the section 105, and the diameter d₁₀₀ of the mixingsection 120 may be, for example, between 0.81 and 0.99, especially(90±9) %. This ratio d₁₀₁/d₁₀₀ is preferably 0.9.

A ratio d₁₀₃/d₁₀₀ between the diameter d₁₀₃ of the inlet 113 for thecombustion air V and the diameter d₁₀₀ of the mixing section 120 may be,for example, between 0.9 and 1.3, especially (110±20) %. This ratiod₁₀₃/d₁₀₀ is preferably 1.1.

A ratio y/d₁₀₀ of the distance y between a flow discharge-side end 124of the guide device 122 and the outlet 115 for the mixture of gas andcombustion air and the diameter d₁₀₀ of the mixing section 120 may be,for example, between 0.5 and 2.5.

In the exemplary embodiment of FIG. 3, the flow discharge-side end 124of the guide device 122 lies upstream of the outlet 115 for the mixtureM, specifically exactly at the end of the mixing section 120 accordingto FIG. 3, however, this is not required.

The flow discharge-side end 124 of the guide device 122 may terminateflush with the end of the mixing section 120 according to FIG. 3. Theflow discharge-side end 124 of the guide device 122 can also liedownstream (or even upstream in some cases) of the end of the mixingsection 120.

In each case, the combustion air of the secondary flow V₂, issuing fromthe combustion air inlet 113, may be mixed downstream of the mixingsection 120 in the region of the diffuser section 121 with the mixtureof the gas G and the combustion air of the primary flow V₁.

The flow inlet-side end 123 of the guide device 122 may lie exactly atthe inlet 113 of the combustion air and terminate flush with thecombustion air inlet 113, as shown in exemplary embodiment of FIG. 3,but this is not required. It is also possible that the flow inlet-sideend 123 of the guide device 122 lies downstream (or even upstream) ofthe combustion air inlet 113.

The flow discharge-side end of the section 104 of the guide device 122,and therefore the flow inlet-side end of the section 105 of the guidedevice 122, may terminate flush with the start of the mixing section 120according to FIG. 3, but again, this is not required. For example, theflow discharge-side end of the section 104 of the guide device 122, andtherefore the flow inlet-side end of the section 105 of the guide device122, can lie downstream or upstream of the start of the mixing section120.

In the exemplary embodiment of FIG. 3, the guide device 122 has acircular contour in cross section. In contrast to this, it is alsopossible that the guide device 122 has an oval, elliptical, or any othersuitable shape contour in cross section.

In the exemplary embodiment of FIG. 2, the guide device 22 provides atype of bypass to the contraction section 19 in which the secondary flowV₂ is neither contracted nor expanded. In the exemplary embodiment ofFIG. 3, on the other hand, the secondary flow V₂ is expanded in thesection 104 of the guide device 122, whereas in parallel to this theprimary flow V₁ is contracted in the contraction section 119. With bothexemplary embodiments, a broader modulation range can be realized, andthis while maintaining a high modulation quality. At the same time, alow flow resistance or throughflow resistance may be achieved.

LIST OF DESIGNATIONS

-   10, 110, 10′ Mixing device-   11, 111, 11′ Housing-   12, 112, 12′ Venturi device-   13, 113, 13′ Combustion air inlet-   14, 114, 14′ Gas inlet-   15, 115, 15′ Outlet-   16, 116, 16′ Annular gap-   17, 117, 17′ Inlet funnel-   18, 118, 18′ Discharge funnel-   19, 119, 19′ Contraction section-   20, 120, 20′ Mixing section-   21, 121, 21′ Diffuser section-   22, 122 Guide device-   23, 123 Flow inlet-side end-   24, 124 Flow discharge-side end-   104 Section-   105 Section-   V Combustion air-   V₁ Primary flow-   V₂ Secondary flow-   G Gas-   M Mixture of gas and combustion air

What is claimed is:
 1. A mixing device for mixing gas and combustion airfor a gas burner, the mixing device having a housing that defines acombustion air inlet, a gas inlet, and an outlet, the mixing devicecomprising: a venturi which serves to mix the combustion air and thegas, wherein the venturi includes a contraction section, a mixingsection and a diffuser section; and a guide device positioned inside theventure, the guide device divides combustion air which enters the mixingdevice via the combustion air inlet of the housing into a primary flowand a secondary flow, wherein the combustion air of the primary flow,issuing from the combustion air inlet of the housing, is fed to themixing section via the contraction section and is mixed with the gas toform a first mixture, and wherein the combustion air of the secondaryflow, issuing from the combustion air inlet of the housing, is mixeddownstream of the mixing section in the region of the diffuser sectionwith the mixture of the gas and the combustion air of the primary flowto form a second mixture.
 2. The mixing device of claim 1, wherein theguide device is positioned concentrically in the venturi.
 3. The mixingdevice of claim 1, wherein the guide device has a cylindrical shape. 4.The mixing device of claim 3, wherein a ratio between a diameter of theguide device and a diameter of the mixing section of the venturi isbetween 0.05 and 0.55.
 5. The mixing device of claim 3, wherein a ratiobetween a length of the guide device and a diameter of the guide deviceis between 5 and
 15. 6. The mixing device of claim 3, wherein a ratiobetween a length of the contraction section of the venturi and adiameter of the mixing section of the venturi is between 0.5 and 1.5. 7.The mixing device of claim 3, wherein a ratio between a length of thediffuser section of the venturi and a diameter of the mixing section ofthe venturi is between 2 and
 6. 8. The mixing device of claim 3, whereina ratio of a distance (x) between a flow discharge-side end of the guidedevice and the outlet of the housing, and a diameter of the mixingsection of the venturi is between 0 and
 2. 9. The mixing device of claim3, wherein: a ratio between a diameter of the guide device and adiameter of the mixing section of the venturi is between 0.05 and 0.55;a ratio between a length of the guide device and the diameter of theguide device is between 5 and 15; a ratio between a length of thecontraction section of the venturi and the diameter of the mixingsection of the venturi is between 0.5 and 1.5; a ratio between a lengthof the diffuser section of the venturi and the diameter of the mixingsection of the venturi is between 2 and
 6. 10. The mixing device ofclaim 9, wherein a ratio of a distance (x) between a flow discharge-sideend of the guide device and the outlet of the housing, and the diameterof the mixing section of the venturi is between 0 and
 2. 11. The mixingdevice of claim 1, wherein the guide device is of a tubular design, andincludes a truncated cone-shaped section which widens out in the flowdirection and a cylindrical section which adjoins the truncatedcone-shaped section downstream.
 12. The mixing device of claim 11,wherein a ratio between a length of the truncated cone-shaped section ofthe guide device and an upstream diameter at an upstream end of thetruncated cone-shaped section of the guide device is between 2 and 6.13. The mixing device of claim 11, wherein a ratio between a length ofthe cylindrical section of the guide device and an upstream diameter atan upstream end of the truncated cone-shaped section of the guide deviceis between 1 and
 3. 14. The mixing device of claim 11, wherein a ratiobetween an upstream diameter at an upstream end of the truncatedcone-shaped section of the guide device and a downstream diameter at adownstream end of the truncated cone-shaped section of the guide deviceis between 0.25 and 0.75.
 15. The mixing device of claim 11, wherein aratio between a downstream diameter at a downstream end of the truncatedcone-shaped section of the guide device and a diameter of the mixingsection of the venturi is between 0.81 and 0.99.
 16. The mixing deviceof claim 11, wherein a ratio of a distance (y) between a flowdischarge-side end of the guide device and the outlet of the housing,and a diameter of the mixing section of the venturi is between 0.5 and2.5.
 17. The mixing device of claim 11, wherein: a ratio between alength of the truncated cone-shaped section of the guide device and anupstream diameter at the upstream end of the truncated cone-shapedsection of the guide device is between 2 and 6; a ratio between a lengthof the cylindrical section of the guide device and the upstream diameterat the upstream end of the truncated cone-shaped section of the guidedevice is between 1 and 3; a ratio between the upstream diameter at theupstream end of the truncated cone-shaped section of the guide deviceand a downstream diameter at a downstream end of the truncatedcone-shaped section of the guide device is between 0.25 and 0.75; and aratio between the downstream diameter at the downstream end of thetruncated cone-shaped section of the guide device and a diameter of themixing section of the venturi is between 0.81 and 0.99.
 18. The mixingdevice of claim 17, wherein a ratio of a distance (y) between a flowdischarge-side end of the guide device and the outlet of the housing,and the diameter of the mixing section of the venturi is between 0.5 and2.5.
 19. A mixing device for mixing gas and combustion air for a gasburner, the mixing device having a housing that defines a combustion airinlet, a gas inlet, and an outlet, the mixing device comprising: aventuri that includes a contraction section, followed by a mixingsection, followed by a diffuser section; the gas inlet of the mixingdevice is in fluid communication with the mixing section of the venturito provide gas to the mixing section of the venturi; a guide devicehaving an outer wall that defines a lumen that extends through the guidedevice from an upstream end of the guide device to a downstream end ofthe guide device, wherein the guide device is positioned in the venturiand passes through at least part of the mixing section of the venturesuch that an outlet of the guide device is downstream of an inlet of themixing section; and the guide device dividing combustion air that entersthe combustion air inlet of the mixing device into a primary flow, whichflows between the outer wall of the guide device and an inner wall ofthe venturi, and a secondary flow, which flows through the lumen of theguide device.
 20. A mixing device for mixing gas and combustion air fora gas burner, the mixing device having a housing that defines acombustion air inlet, a gas inlet, and an outlet, the mixing devicecomprising: a venturi that includes a contraction section, followed by amixing section, followed by a diffuser section; the gas inlet of themixing device is in fluid communication with the mixing section of theventuri to provide gas to the mixing section of the venturi; a guidedevice having an outer wall that defines a lumen that extends throughthe guide device from an upstream end of the guide device to adownstream end of the guide device, wherein the upstream end of theguide device is positioned upstream of the mixing section of the venturiand the downstream end of the guide device is downstream of the mixingsection of the venturi; and the guide device dividing combustion airthat enters the combustion air inlet of the mixing device into a primaryflow, which flows between the outer wall of the guide device and aninner wall of the venturi, and a secondary flow, which flows through thelumen of the guide device.